(1) Field of the Invention
The present invention relates to a thermoplastic polymer composition reinforced with natural fibers such as cellulose or other fillers and to a process for manufacturing the composition. The process for making a naturally derived fiber-reinforced thermoplastic polymer composition comprises melting a high melting temperature thermoplastic polymer, mixing the melted thermoplastic polymer with an organic or inorganic salt to reduce the melting temperature of the melted thermoplastic polymer to a melting temperature which does not degrade the natural fibers, and then adding the naturally derived fibers to produce the naturally derived fiber-reinforced thermoplastic polymer composition. The naturally derived fiber-reinforced thermoplastic polymer composition can then be melted at the reduced melting temperature to manufacture a plurality of articles.
(2) Description of Related Art
Because of their ability to be readily molded into three-dimensional objects, plastics are the most commonly chosen engineering materials in any industry. These plastics can be easily tailored to have a wide range of physico-mechanical properties. With the addition of high strength and modulus reinforcements and filler materials like fibers, plastics can be designed for structural applications. Glass fibers are the workhorse for polymer/fiber composite materials. Glass fiber reinforced plastic composites provide the desired stiffness and toughness required for structural applications. Although there are many advantages of the glass fiber composites, there are certain disadvantages also which include wear and abrasion of processing equipment, high cost and low recycle potential.
With increasing environmental concerns, natural fibers have been found to be a potential reinforcement and filler for both thermoset and thermoplastic composites. Natural fibers are plant based agricultural products and are obtained either from the leaf, seed or stalk of a plant. These natural fiber reinforced bio-composites can find a wide variety of applications mainly in the automotive and building construction industries. Long natural fibers, like flax, kenaf, and hemp are finding increasing use, mainly as reinforcement materials in automotive composites. Economics, weight reduction, strength, durability, compatibility, easy recyclability and environmental friendliness are the driving forces for the use of natural fibers in composites replacing glass fibers.
The residue produced when glass-fiber reinforced plastic composites are burned is made up of large numbers of very small glass filaments which cannot be broken down using ecologically compatible methods. Furthermore, the glass dust generated can cause health risks. Plastics reinforced with natural fibers, on the other hand, are harmless from both an ecological and occupational health viewpoint. When the natural fibers are burned they release only the amount of carbon dioxide which the plant absorbed when it was growing and therefore the use of plant material is CO2 neutral, that is there is no increase in greenhouse gases. In light of all the above, research has been undertaken to reinforce the engineering thermoplastics with the natural fibers replacing the existing glass fibers.
Extrusion and injection molding are the two very important processing techniques employed in any engineering industry involved with plastics and composites. Researchers have successfully used natural fibers as reinforcing materials replacing the glass fibers. But they have been forced to adhere to low temperature melting thermoplastic-natural fiber composites as the natural fibers start to degrade thermally at or above 200° C. Thermal degradation of natural fibers results in poor physico-mechanical properties and discoloration of the fibers. Reinforcing high temperature melting thermoplastics which melt above 200° C. with natural fibers has turned out to be a challenging task. Hence, a processing technique where one can depress the melting temperature of the high temperature melting thermoplastic to below 200° C. would be an ideal solution for this problem.
Very few attempts have been made to develop a process for producing thermoplastic polymer compositions reinforced with natural fibers from high temperature melting thermoplastic polymers because the natural fibers degrade at or below the melting temperatures of these plastics. The only process known for producing high temperature melting thermoplastic polymer compositions reinforced with natural fibers the inventors are aware of is disclosed in U.S. Pat. No. 6,270,883 and U.S. Application Publication No. 2002/0000683 A1, both to Sears et al. The high temperature melting thermoplastic polymer compositions were prepared by compounding a high temperature melting thermoplastic polymer and a natural fiber in an extruder in which all the zones of the extruder had been heated to an initial temperature above the melting temperature of the plastic. After running the polymer and the fiber for a while through the extruder, the temperature of the extruder from the mid-zone to the end-zone was lowered to a temperature well below 200° C. This enabled the polymer, which had been melted in the initial high temperature zones, to still flow through the low temperature mid- and end-zones thus, making it possible to now add natural fibers to the melted polymer without danger of exposing the fibers to the high temperatures needed to melt the polymer. Thereafter, once the process had reached an equilibrium temperature from the mid-zone to the end-zone, the composition was collected from the end of the extruder. The extruded composition was then injection molded at a temperature above the melting temperature of the polymer. Therefore, even though the fibers had been incorporated into the polymer at a temperature low enough to preserve the integrity of the fibers, injection molding the composition at the melting temperature of the polymer causes the natural fibers to degrade. Thus, the original strength of reinforcing natural fiber in the composition is lost.
There is an extensive body of literature on the interaction of the various metal halides with polyamides like nylon. For example, it was discovered that the addition of metal halides depressed the melting temperature of nylon. Lithium chloride, lithium bromide, and copper chloride were found to be effective in reducing the melting temperature of polyamides. It was also discovered that compounding polyamides with inorganic salts such as metal halides imparted desirable qualities to the compounded product such as elimination of voids and improving the elastic modulus which had already been established in laboratory scale experiments wherein the blending of polyamides and metal halides was done by dissolving them in a solvent and then extracting the solvent.
Several U.S. patents disclose processes for lowering the melting temperature of polyamides. U.S. Pat. No. 4,092,301 to Russo et al. discloses a process for making low melting point polyamides in which a lactam is polymerized in a mixture comprising an alkaline anionic catalyst, an anionic activator, and either LiCl or LiBr to make a polymer. U.S. Pat. No. 4,619,962 to Sato discloses a low melting point thermoplastic composition comprising a polyamide, a halide such as LiCl, a synthetic rubbery polymer, and an oxide or hydroxide of magnesium, calcium, barium, or zinc, or a peroxide of calcium or zinc and a process for producing the same. U.S. Pat. Nos. 4,481,354 and 4,588,797, both to Curatolo et al. disclose a process and composition for making low melting point nylon compositions comprising contacting a nylon composition or mixture thereof with a lithium halide and an organic sulfone. However, none of the above U.S. patents suggest that the low melting point polyamides can be used for making thermoplastic compositions reinforced with natural fibers.
Despite the knowledge that particular metal halides reduce the melting temperature of polyamides, the inventors are not aware that the knowledge has ever been correlated with a process for producing thermoplastic polymer compositions reinforced with natural fibers wherein using a polyamide in which the melting point has been reduced enables the integrity of the natural fibers to be preserved. Thus, until the present invention, it was not possible to make high temperature melting thermoplastic polymer compositions reinforced with natural fibers.